The present invention relates to a method for the open-loop control or closed-loop control of at least one operating parameter influencing the aging state of an electric energy accumulator.
Hybrid vehicles are equipped with a high-voltage accumulator (battery) that stores electric energy for supplying an electric drive motor. Such high-voltage accumulators are expensive and, if possible, should therefore be designed to last for the entire service life of the vehicle. It is known that batteries are subject to physical aging, which is a function of a plurality of parameters, such as the operating temperature of the battery over time, the number of charging and discharging cycles, the charging and/or discharging swings, etc.
From JP 111 655 40, for example, it is known to protect a battery from excessive stressing and therefore excessively fast aging by a voltage limitation or an overcharging or undercharging protection.
Methods for cooling or for limiting the current of batteries are known from CN 1012 22077 or JP 2005012929. The technical background of the present invention also includes DE 10 2008 009 568.0, U.S. Pat. No. 5,672,951, JP 6140080, WO 07114410, JP 2007216707, JP 2005125916, JP 2008196852, U.S. 2005/001625, DE 102007023901, DE 102007012127, as well as JP 2004186087.
The state of the art attempts to avoid damage to the electric energy accumulator by defining constant or slightly dynamic limit values, for example, for the peak power and the constant power, the permissible voltage or the charging condition, etc. However, electric energy accumulators age also within fixedly or dynamically predefined operating limits, particularly during the charging, the discharging, and as a result of high thermal stress.
It is an object of the invention to indicate a method for the open-loop or closed-loop control of at least one operating parameter that influences the aging state of an electric energy accumulator, which makes it possible to utilize the energy accumulator as best possible for a predefined target service life.
This and other objects are achieved by a method for open-loop control or closed-loop control of at least one operating parameter of an electric energy accumulator influencing the aging state of the electric energy accumulator. The method determines the actual aging state of the electric energy accumulator. It compares the actual aging state with a target aging state predefined for the momentary age of the energy accumulator. And, it restricts an operating parameter range permitted for the at least one operating parameter if the actual aging state is worse than the target aging state.
The basic principle of the invention consists of monitoring the aging state of an energy accumulator and comparing it with a predefined target aging state and, “if required”, restricting the permissible value range or the permitted operating parameter range of at least one operating parameter of the energy accumulator that can be influenced. The goal is to maintain the functioning capacity of the energy accumulator to the end of its predefined target service life; i.e., so that the energy accumulator meets predefined minimum criteria to the end of the predefined target service life.
The term “electric energy accumulator” especially comprises batteries or accumulators for electric vehicles or hybrid vehicles.
The restriction of the permissible value or of the permitted operating parameter range takes place only if required, i.e. when the actual aging state of the energy accumulator is worse than a target aging state predefined or expected for the “momentary service life” of the energy accumulator. In the present patent application, the term “State of Health” or the corresponding acronym “SoH” will also be used instead of the term “aging state”.
According to a further aspect of the invention, the actual aging state of the energy accumulator is determined as a function of the internal resistance of the energy accumulator and/or as a function of the capacity of the energy accumulator.
The actual aging state of the energy accumulator preferably is determined as a function of a measured actual internal resistance and of the determined actual capacity of the energy accumulator. Methods for determining the capacity of an energy accumulator are known from the state of the art (for example, EP 1962 099 A2) and therefore do not have to be explained here in detail.
The internal resistance as well as the capacity, or a linking of both characteristics, can be used as indicators, a measurement or as important defining quantities for the aging state of an internal resistance. The internal resistance of an energy accumulator will rise as the aging state deteriorates. On the other hand, the capacity of an electric energy accumulator will decrease as the aging state deteriorates.
The aging of an electric energy accumulator is decisively influenced by its thermal stressing which, in turn, is a function of the ambient temperature, the heat generated during the charging and discharging, and the cooling of the electric energy accumulator.
In the case of the “SoH-based closed-loop control” provided according to the invention, for example, the dependency between the depth of discharge or DoD and the cyclical stressing of the energy accumulator can be utilized as a “control variable”, if necessary, for slowing down the rate of aging. “Energy swings” during the charging and discharging can thereby be influenced, for example. It is known that, in the case of smaller energy swings, the degrading of the accumulator and thus the rate of aging is therefore less than in the case of greater energy swings.
According to the invention, the open-loop control or closed-loop control of the at least one operating parameter influencing the aging state of the electric energy accumulator takes place as a function of the so-called “relative deviation” dSoHrel of the actual aging state of the energy accumulator from the aging state expected for the momentary age of the energy accumulator, which aging state is also called herein a “target aging state.” This naturally does not mean that this is an aging state to be endeavored but that the actual aging state of the energy accumulator should at most be as “bad” as the target aging state, but preferably should be better.
According to the invention, the relative deviation dSoHrel is used as the input quantity of a closed-loop control device or open-loop control device, which, if required, limits the permissible value range of at least one operating parameter influencing the aging state of the energy accumulator in order to ensure a certain minimum functionality until the end of a predefined target service life of the energy accumulator.
According to a further aspect of the invention, the at least one operating parameter is the charging condition of the energy accumulator, also called “state of charge” or “SoC”.
It may be provided that the maximal charging limit, up to which the energy accumulator is permitted to be charged, is lowered by the open-loop control or closed-loop control if the actual SoH falls below the target state of aging SoHtarget. By lowering the maximal charging limit, the possible discharging depth is reduced, which reduces the rate of aging of the energy accumulator. Although a lowering of the maximal charging limit reduces the effectively available amount of energy stored in the energy accumulator, this is intentionally accepted in order to ensure that a predefined target service life can be reached.
According to a further aspect of the invention, it may be provided that the rate of change, i.e. the gradient by which the maximal charging limit is reduced, is limited in order to make it possible for the customer or user of the energy accumulator to gradually become accustomed to a reduction of the maximal charging limit, rather than have an abrupt experience.
In addition or as an alternative, for lowering the maximal charging limit SoCmax, the permissible lower charging limit SoCmin, to which the energy accumulator is permitted to be discharged, may be raised when there is a falling below the target aging state, in order to always provide the electric energy required for a cold start of the vehicle also in the case of an aged energy accumulator. It may be provided that the raising of the minimal charging limit SoCmin takes place independently of the intensity of the aging and of the SoH closed-loop control above SoCmax.
According to the invention, an operating strategy may be provided, which additionally expands the methods known from the state of the art for protecting the energy accumulator. Electric accumulators age particularly when currents are very high or in the case of a high power output. In order to limit or control the aging of the electric accumulator, it may be provided that, as a function of the relative deviation dSoHrel of the actual aging state SoHact from the target aging state SoHtarget, the maximally permissible or available charging and discharging power is restricted. For this purpose, predefined power limits, which can also be predefined as dynamic power limits, i.e., for example, time-dependent power limits or power limits dependent on the voltage level, are multiplied by a weighting factor between zero and one. In order to design the intervention of the open-loop control or closed-loop control such that it will not be noticeable to the customer or user, the maximal gradient of the weighting factor as well as its lower limit may be restricted. If required, power peaks will be avoided as a result of this intervention. The thermal stressing of the accumulator will correspondingly also be reduced.
As an additional intervention possibility, it may be provided that temperature limits are dynamically adapted. The static limit values known from the state of the art define as of which temperature the accumulator is cooled or as of which temperature the cooling will be deactivated again. Such static temperature limits keep the accumulator temperature below a certain value. However, if the aging of the energy accumulator is significantly determined by the thermal stressing, for example, if a vehicle is used in a “hot-climate country”, an adaptation of the temperature limits may be useful. These temperature limits may be changed, especially lowered, as a function of the actual aging state of the energy accumulator. As a result of lowering the temperature limits, the average temperature of the electric accumulator will be lower, and the cooling time of the electric accumulator will be shortened when the vehicle is parked. In order to ensure that the closed-loop control will not be noticed by the customer, it may also be provided here that the gradient of the lowering of the temperature limits is restricted.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.